Solid-state lighting devices such as light-emitting diodes (LEDs) are increasingly used in both consumer and commercial applications. Advancements in LED technology have resulted in highly efficient and mechanically robust light sources with a long service life. Accordingly, modern LEDs have enabled a variety of new display applications and are being increasingly utilized for general illumination and various other applications, often replacing incandescent and fluorescent light sources.
LEDs are solid-state devices that convert electrical energy to light and generally include one or more active layers of semiconductor material (or an active region) arranged between oppositely doped n-type and p-type layers. When a bias is applied across the doped layers, holes and electrons are injected into the one or more active layers where they recombine to generate emissions such as visible light or ultraviolet emissions. An LED chip typically includes an active region that may be fabricated, for example, from epitaxial layers of silicon carbide, gallium nitride, gallium phosphide, aluminum nitride, gallium arsenide-based materials, and/or from organic semiconductor materials. Photons generated by the active region are initiated in all directions.
Typically, it is desirable to operate LEDs at the highest light emission efficiency possible, which can be measured by the emission intensity in relation to the output power (e.g., in lumens per watt). A practical goal to enhance emission efficiency is to maximize extraction of light emitted by the active region in a direction of the desired transmission of light. In a typical LED package 10 illustrated in FIG. 1, a single LED chip 12 is mounted on a reflective cup 14 by means of a solder bond or conductive epoxy. One or more wire bonds 16 can connect the ohmic contacts of the LED chip 12 to leads 18A and/or 18B, which may be attached to or integral with the reflective cup 14. The reflective cup 14 may be filled with an encapsulant material 20, which may contain a wavelength conversion material such as a phosphor. At least some light emitted by the LED chip 12 at a first wavelength spectrum may be absorbed by the phosphor, which may responsively emit light at a second wavelength spectrum. The entire assembly is then encapsulated in a clear protective resin 22, which may be molded in the shape of a lens to direct the light emitted from the LED chip 12 in a direction 24 that is predominantly perpendicular to a surface of the reflective cup 14 where the LED chip 12 is mounted.
FIG. 2 shows another typical LED package 26 in which one or more LED chips 28 can be mounted onto a carrier such as a printed circuit board (PCB) carrier, substrate, or submount 30. A metal reflector 32 mounted on the submount 30 surrounds the one or more LED chips 28 and reflects light emitted by the one or more LED chips 28 in a direction 34 predominantly perpendicular to a surface of the submount 30 on which the one or more LED chips 28 is mounted. One or more wire bond connections 36 are made between ohmic contacts on the one or more LED chips 28 and electrical traces 38A, 38B on the submount 30. The mounted one or more LED chips 28 are then covered with an encapsulant 40, which may provide environmental and mechanical protection to the LED chip(s) 28 while also acting as a lens.
FIG. 3 shows another typical LED package 42 in which an LED chip 44 can be mounted on a submount 46 with a hemispheric lens 48 formed over it. The LED chip 44 can be coated by a conversion material that can convert all or most of the light from the LED chip 44. The hemispheric lens 48 is arranged to reduce total internal reflection of light. As a result, an increased amount of LED light that reaches the surface of the lens 48 transmits through the lens 48 on a first pass. Additionally, the lens 48 can be useful for directing light emission from the LED chip 44 in a desired emission pattern toward a direction 50 that is predominantly perpendicular to a surface of the submount 46 on which the LED chip 44 is mounted.
FIG. 4 shows another typical LED package 52 that is arranged to have primary light emission in a direction 54 that is predominantly parallel to a surface 56 on which the LED package 52 is mounted. The LED package 52 typically includes an LED chip 58 mounted on a submount 60. In order to have the primary light emission in the direction 54, the LED package 52 is mounted on its side in reference to the chip orientation, which is sometimes referred to as a sidelooker or side view LED. In this manner, the direction 54 is predominantly perpendicular to a surface of the submount 60 on which the LED chip 58 is mounted. In this regard, each of the LED packages 10, 26, 42, and 52 of FIGS. 1-4 has a predominant emission direction that is centered along or near the normal of the LED chip mounting surface as well as epitaxial layers within the LED chips.
FIG. 5 is a plot representing a typical spatial distribution for an emission pattern of a conventional LED package. The x-axis represents a viewing angle in degrees as measured from a direction normal to a primary emission face of an LED chip of the conventional LED package. For example, 0° is perpendicular to the primary emission face. The y-axis represents relative luminous intensity for a given viewing angle. As illustrated, the highest luminous intensity percentages are centered along a direction that is predominantly perpendicular to the primary emission face of the LED chip, which is also predominantly perpendicular to the surface on which the LED chip is mounted. In this manner, the conventional LED packages as previously described, are all configured to have the primary emission faces of the LED chips oriented predominantly perpendicular to primary emission directions of the LED packages.
The art continues to seek improved LEDs and solid-state lighting devices having reduced optical losses and providing desirable illumination characteristics capable of overcoming challenges associated with conventional lighting devices.